About 60% of newborns become jaundiced, and about 6% develop total serum bilirubin (TSB) levels at which the American Academy of Pediatrics (AAP) recommends treatment with phototherapy be considered. Decisions about whether to use phototherapy in these newborns should be based upon the risk that the TSB will rise to a potentially dangerous level (> 25 mg/dL) in the absence of treatment, and the efficacy of phototherapy and other interventions at reducing that risk, balanced against their costs and potential harms. Such data, however, are not generally available because these newborns are usually treated well before their TSB reaches 25 mg/dL. In the Northern California Kaiser Permanente Medical Care Program (NC KPMCP) in 1995-96, however, only 27% to 77% of newborns born in different facilities received phototherapy at levels recommended by the AAP. This interfacility variation in adherence to guidelines and the NC KPMCP'S excellent data systems enable a study to estimate the risk of newborns with known hyperbilirubinemia developing a TSB > 25 mg/dL and the efficacy of treatments at reducing that risk. We propose a nested case-control study within a cohort of newborns > 2000 g birth weight and > 34 weeks gestation born 1995-2004 (N~285,000) with TSB levels of 17-22.9 mg/dL at > 48 hours of age (N-16,140). Cases (N~83) will be those who subsequently developed a TSB ^ 25 mg/dL. Controls will be randomly selected in a 4:1 ratio, frequency matched to cases by TSB in 1 mg/dL strata. Predictor variables, including gestational age, breast-feeding, and most recent changes in weight and TSB levels, will be analyzed using multivariate techniques to control confounding, and combined with the cohort size to estimate absolute risk. Estimated efficacy of treatments, including timing and type of phototherapy and feeding changes, will be combined with these absolute risks to allow estimation of numbers needed to treat for different treatments and infants at various levels of baseline risk. Finally, NC KPMCP databases will be used to explore possible effects of phototherapy on future outpatient service utilization. The results of this study will help make neonatal jaundice guidelines and treatment decisions more evidence based. [unreadable] [unreadable] [unreadable]